106 MEMOIR ON BOSTON HARBOR. 



6. As, in a channel of irregular boundaries, the greatest strength of the current is 

 found to be, according to circumstances, on one side or the other, leaving on the side op- 

 posite to it an eddy, or a space of still water, or even a current running in the inverse 

 direction, and coming in conflict at the turning-point with the main stream, (a counter- 

 current as it is called,) and as these conditions are favorable to deposit ; •• and 



7. As the deposits caused by such irregularities tend continually to divert the current 

 from its proper channel, and may even, as in the well-known case of the Mystic, com- 

 pletely alter the channel, closing the old and opening a new one ; so we are led to ob- 

 serve a sixth fundamental principle; which is, — 



That the limit or boundary lines of the channel should be as regular as possible, co- 

 inciding with the natural course of the waters ; and it would even be desirable to have 

 the sides of the channel steep, if practicable, because, — 



8. The velocity of the current is very much diminished on sides of gradual slope, 

 and the suspended matter, therefore, is carried there to be dej)osited. 



The preceding statement of the importance of adopting such a form of channel as 

 will allow a natural flow to the water, of making the bounding lines of the channel reg- 

 ular, and of avoiding obstructions to the current, however comprehensive it may seem to 

 be, still leaves room for the introduction of another fundamental principle similar to those 

 just stated, but derived from the following distinct considerations : — 



9. It most commonly happens that, in tidal harbors, the channel or natural course of 

 the water from the receptacle, or river-basin answering the purpose of the receptacle, is 

 circuitous and indirect. Where the natural channel by which the back-water so passes 

 to the sea is direct, as in the inner harbor of New York from the North River to the 

 Narrows, it requires no artificial construction to improve the form of the channel, or to 

 change its direction. But in the ordinary cases first mentioned, in which Boston Harbor 

 is embraced, improvement of form and change of direction may be necessary. 



10. Now, since the water does not rebound as a mass from an opposing wall or sur- 

 face, like an elastic body, (a fact 1 should consider it wholly superfluous to mention, if the 

 opposite opinion had not been maintained by respectable engineers,) but, on the contrary, 

 its currents follow adhesively the changes in the form and direction of that surface; 



11. And since we observe in nature, that, wherever one of the bounding sides of a 

 channel retreats back from the former line of direction, there is invariably a deposit oppo- 

 site the retreating point, we are conducted to a seventh fundamental principle ; which is, — 



That, where one of the sides of a channel is altered by an artificial construction, the 



* Abbe Mann. 



